Post on 08-Sep-2018
Prototype Study of Biofilm Processes with Mobile Carriers for Sewage Treatment
by
Dr. Anthony MA Principal Consultant
Environmental Management Division
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DSD R&D Forum 2012
Wastewater Treatment Session
28 November 2012
Content
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• Introduction – Biofilm Processes
• Prototype Study
• Study Results
• Comparison between FA and IFAS
• Comparison between Treating Raw Sewage and CEPT Effluent
• Comparison between IFAS and MBBR
• Summary of Study Results
• Potential Application in HATS 2B
Biofilm Processes (Attached Growth)
With
Fixed-bed
Media
With
Moving-bed
Media
Rotating
Biological
Contactor
(RBC)
Trickling Filter
Submerged
Aerated Filter
(SAF)
Biological
Aerated Filter
(BAF)
Moving Bed
Bioreactor
(MBBR)
Integrated Fixed
Film Activated
Sludge
(IFAS)
Introduction – Biofilm Processes
Introduction – Biofilm Media
• Fixed-bed Media
• Moving-bed Media
• Microorganisms grow and develop on the
surface of MBBR carrier (biofilm)
• Decomposition of organics take place in
biofilm
• As biofilm grows thicker, it will slough off
from the carrier and settled as sludge
Biological
growth
Air
Wastewater
Aerobic Biofilm
Plastic Carrier
Surface
Anaerobic Biofilm
Oxygen
Organic matters
and pollutants
Carbon dioxide
Biodegraded pollutants
5
Introduction – Biofilm Processes
Introduction – Prototype Study
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• CEPT effluent from the SCISTW needs further treatment under the HATS 2B to
meet the future WQO
• In view of the space constraints in SCISTW, DSD has commissioned HKPC to
evaluate the applicability and performance of biofilm processes with mobile
carriers on treating the CEPT effluent
• The prototype reactors were housed in a 20-ft container in SCISTW
• The study was conducted between 12 Oct 2010 and 28 Jun 2012
Schematics of Biofilm Reactors
• Integrated Fixed Film Activated Sludge (IFAS) – hybrid system with denitrification
• Fully-Aerated (FA) System – hybrid system without denitrification
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• Moving Bed Biofilm Reactor (MBBR) System – pure attached-growth system
Objectives of Prototype Study
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• To assess the effectiveness of using biofilm reactors to treat the CEPT effluent
and raw sewage
• To operate the prototypes at stressed conditions so as to determine the
maximum loading/shortest HRT that can be accepted by the biofilm reactors to
meet generic effluent criteria:
• BOD5 ≤ 20 mg/L
• TSS ≤ 30 mg/L
• Ammonia-nitrogen ≤ 5 mg/L @18°C
• To assess the denitrification performance of the biofilm reactors (based on TN ≤
20 mg/L)
• To estimate the sizes of the biofilm reactors for SCISTW for illustration
Introduction
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FA IFAS MBBR
Volume of tank 150 L 150 L 150 L
Volume of anoxic zone N/A 45 L (30%) 60 L (40%)
Volume of aerobic zone 150 L 105 L (70%) 90 L (60%)
Volume of carriers in anoxic zone N/A N/A 30 L (50%)
Volume of carriers in aerobic zone 90 L (60%) 63 L (60%) 59 L (65%)
Mobile carriers AnoxKaldnes K3
Specific biofilm surface area (in bulk) 500 m2/m
3
Biofilm surface area used in anoxic zone N/A N/A 15 m2
Biofilm surface area used in aerobic zone 45.0 m2 31.5 m
2 29.5 m
2
Range of Flow rate 4001,700 L/day
DO setting in aerobic zone 3 4 mg/L
System Parameters of FA, IFAS and MBBR
• At moderate HRT (~6.5 hr), FA and IFAS could fully meet the effluent criteria of
TCOD, TSS and NH3-N when treating both raw sewage and CEPT effluent at 23°C.
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Comparison between FA and IFAS
Comparison between FA and IFAS
1) Hybrid FA system without pre-denitrification suffered from pH drop problem after
nitrification when treating CEPT effluent
2) Hybrid system with pre-denitrification (IFAS) has the merits of TN removal and
supplementation of alkalinity consumption and oxygen requirement
3) Limitation of IFAS – when the reactor MLSS is further reduced, pre-denitrification
performance may deteriorate.
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Prototype
System
Operating
Temp.
Min. HRT (hr) % Reduction
in Min. HRT Raw Sewage CEPT Effluent
IFAS 23
oC 4.2 2.4 43%
18oC 5.1 – 5.6 3.1 – 3.2 39 – 42%
MBBR 23
oC 6.2 3.8 39%
18oC 7.2 – 7.8 4.5 – 4.9 38%
Shorter Min HRT in CEPT Effluent
at 23°C and 18°C
• Minimum HRT required for MBBR and IFAS to treat CEPT effluent were
impressively short
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Biomass Overgrowth
under High Loading Rates of Raw Sewage
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Normal Condition Biomass Overgrowth Normal Condition Biomass Overgrowth
MBBR IFAS
• Flourishing growth and thick layer of heterotrophic biomass
• Hinder oxygen from penetrating into the biofilm of nitrifiers
• Suppressed growth and activities of nitrifiers
• Poor ammonia removal performance
• Long restoration period
Lower Organic Content in CEPT Effluent
Favouring Ammonia-N Removal
• CEPT effluent had lower COD and BOD5 contents but comparable levels of NH3-N
and TKN as raw sewage
• When treating CEPT effluent, higher NLR could be achieved as corresponding
OLR was not as high as on treating raw sewage.
• CEPT effluent was more favourable to higher NLR and shorter HRT.
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TN Removal of IFAS and MBBR
on treating CEPT Effluent
• Effluent TN of IFAS and MBBR on treating CEPT effluent could comply with the
effluent criteria of TN ≤ 20 mg/L, but not so good as treating raw sewage because
of less carbon source to facilitate denitrification.
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COD and BOD5 Removal
• Comparable and satisfactory treatment performance in COD and BOD5 removal
• MBBR showed slightly better removal on COD and BOD5
• Both systems started deteriorating under very high loading conditions
• It took time for biomass acclimatization
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Notes: The generic effluent criteria of BOD5 ≤ 20 mg/L.
NH3-N Removal
• NH3-N removal was the
limiting factor in
treatment performance
• Low effluent NH3-N
could be achieved under
favourable operating
condition
• NH3-N removal was
impaired by biomass
overgrowth or under
stressed condition
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• To facilitate higher NLR and shorter HRT
Control OLR within acceptable level to avoid biomass overgrowth
Separate nitrification from carbonaceous removal process (only applicable to MBBR)
Notes: The generic effluent criteria of NH3-N ≤ 5mg/L.
TN Removal and Effluent NO2-N + NO3-N
• Both IFAS and MBBR showed satisfactory denitrification performance. Effluent
TN of both systems ≤ 20mg/L so TN was not a limiting factor
• Effluent NO2-N + NO3-N of both systems was higher on treating CEPT effluent,
showing better denitrification when treating raw sewage.
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Notes: The generic effluent criteria of TN ≤ 20 mg/L.
Effluent SS
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Notes: The generic effluent criteria of TSS ≤ 30mg/L.
• Effluent TSS of MBBR was better than IFAS on treating either raw sewage or
CEPT effluent.
• MBBR had more flexibility on final clarification due to much lower solid loading to
the final clarifiers and no sludge return
MLSS and Sludge Settling Properties
• While SV30 of MBBR maintained at < 40 ml/L when treating CEPT effluent, SV30 of
FA and IFAS fluctuated between 100 – 1000 ml/L.
• IFAS suffered the occurrence of filamentous bacteria (sludge bulking) under high
loading conditions.
• Final clarification for IFAS is relatively difficult and requires longer retention time.
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Minimum HRT of IFAS and MBBR
Minimum HRT achieved by IFAS and MBBR were governed by NH3-N removal efficiency
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Prototype System MBBR IFAS
Operating Temp. (ºC) 23 18 23 18
Raw Sewage 6.2 7.2 – 7.8 4.2 5.1 – 5.6
CEPT Effluent 3.8 4.5 – 4.9 2.4 3.1 – 3.2
Biomass in IFAS and MBBR
1) Substantial attached growth
biomass for both IFAS and
MBBR whereas the
suspended biomass in IFAS
was low.
2) MBBR carrier was more
susceptible to biomass
overgrowth under high OLR
3) MBBR had greater biomass
than IFAS in anoxic zone.
There might be room to
further reduce the anoxic
fraction in MBBR and in turn
reduce the HRT.
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Notes: MLSS controlled at 900 mg/L in IFAS from Phase 2 Part 4.
Relative Amount of Nitrifiers
Relative amount of nitrifiers in the biomass of MBBR and IFAS determined by qPCR analysis carried out
by the research team of Dr. T. Zhang, Department of Civil Engineering, The University of Hong Kong. 28
• Nitrifier population concentrated on biofilm carriers in aerobic zone for both systems.
• When treating CEPT effluent, MBBR actually had more nitrifiers than IFAS, but lower
nitrification efficiency
Nitrification Efficiency
• The lower nitrification efficiency in MBBR than IFAS might possibly due to:
For MBBR under high OLR, heterotrophic biomass grew excessively on top of the
autotrophic biomass on the mobile carriers, the biomass overgrowth could have
affected DO penetration and substrate diffusion to the nitrifiers and impaired the
nitrification activity;
For IFAS, the influent COD was first removed by the suspended growth biomass in
both anoxic zone and aerobic zone of the IFAS reactor, so there was less adverse
effect of “biomass overgrowth” on the nitrification.
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Summary of Study Results
1. At moderate HRT (~6.5 hr), IFAS could fully meet the effluent criteria of BOD, TSS,
NH3-N and TN when treating both raw sewage and CEPT effluent at 23°C.
2. FA system suffered from pH drop problem when treating CEPT effluent whereas
hybrid system with pre-denitrification (IFAS) had the merits of TN removal and
supplementation of alkalinity consumption and oxygen requirement.
3. Minimum HRT of IFAS and MBBR on treating raw sewage and CEPT effluent
(governed by ammonia removal) at 23°C and 18°C were impressively short
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Prototype
System
Operating
Temp.
Min. HRT (hr)
Raw Sewage CEPT Effluent
IFAS 23
oC 4.2 2.4
18oC 5.1 – 5.6 3.1 – 3.2
MBBR 23
oC 6.2 3.8
18oC 7.2 – 7.8 4.5 – 4.9
Summary of Study Results
4. When treating raw sewage under high loading, biofilm carriers are susceptible to
biomass overgrowth. CEPT effluent has lower organic content but comparable
nitrogen content as raw sewage. This led to lower organic loading rate (OLR),
favouring higher nitrogen loading rate (NLR) and shorter HRT.
5. IFAS and MBBR have comparable and satisfactory performance:
– Both systems showed satisfactory denitrification performance
– IFAS had shorter minimum HRT governed by NH3-N removal efficiency
– MBBR showed better COD and BOD5 removal
– MBBR had much lower solid loading to final clarifier and better sludge settling
properties, resulting in better effluent TSS
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Potential Application in HATS 2B
(5 Options)
Treating
CEPT Effluent
IFAS Option
MBBR Basic Option
MBBR Improved Option
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Treating
Raw Sewage
IFAS Option
MBBR Option
• A two-level basement, each level with a maximum floor area of about 130,000 m2,
will be designed for the additional secondary treatment for the HATS 2B. The
average dry weather flow (ADWF) is about 2,441,000 m3/d in HATS 2B.
• Only the areas for the biological reactors and final clarifiers are considered.
• The space requirement of other auxiliary equipment, like pumps, air blowers,
sludge handling, odour control system and other facilities, has not yet been taken
into account.
• Based on the minimum HRT for IFAS and MBBR to treat CEPT effluent and raw
sewage at 18°C.
Design Options for CEPT Effluent
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Mixed Liquor Recycle
CEPT
Effluent Anoxic
Reactor
Hybrid Aerobic
Reactor
Secondary
Clarifier
Final
Effluent
Returned Activated Sludge
Option 1 – IFAS Option
Mixed Liquor Recycle
CEPT
Effluent Anoxic
Reactor
BOD Removal / Nitrification
Reactor DAF
Final
Effluent
Waste
Sludge
Option 2 – MBBR Basic Option
Option 3 – MBBR Improved Option
Mixed Liquor Recycle
CEPT
Effluent Anoxic
Reactor
BOD Removal
Reactor
DAF Final
Effluent
Nitrification
Reactor
Waste
Sludge
Design Options for Raw Sewage
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Mixed Liquor Recycle
Anoxic
Reactor
Final
Effluent
Waste
Sludge
Existing CEPT
Plant
BOD Removal / Nitrification
Reactor
Raw
Sewage
Option 4 – IFAS Option
Option 5 – MBBR Option
Mixed Liquor Recycle
Raw
Sewage Anoxic
Reactor
Hybrid Aerobic
Reactor
Secondary
Clarifier
Final
Effluent
Returned Activated Sludge
Influent CEPT Effluent Raw Sewage
Option Option 1 Option 2 Option 3 Option 4 Option 5
Unit IFAS
Option
MBBR
Basic
Option
MBBR
Improved
Option
IFAS
Option
MBBR
Option
Hydraulic Retention Time of Biological
Reactors (Duty)
hr 3.11 4.49 3.35 5.11 7.20
Estimated floor area of Reactors
(including access for maintenance)
m2 65,200 94,200 70,200 107,100 151,000
Estimated floor area for Final
Clarification
(including access for maintenance)
m2 129,600 60,000 60,000 129,600 Use
existing
CEPT
Plant
Footprint of IFAS and MBBR Options
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• Based on the study results, five design options have been developed for HATS
2B for comparison. In view of space constraints, MBBR options would be more
promising
Further Studies on MBBR Optimization
1) To optimize the MBBR reactor by:
separating the aerobic process into two stages of BOD removal and
nitrification;
raising the DO to 5 mg/L in the nitrification reactor and so increasing the
nitrification rate;
increasing the media fill ratio to 67% or higher, in all bioreactors;
optimizing the volume-fractions and HRT proportions of the anoxic, BOD
removal, and nitrification reactors so that the overall system can attain a
shorter minimum HRT.
2) To improve the MBBR option treating raw sewage by pre-treatment using
compact solids removal technology such as fine mesh sieve filters to removing
particulate material.
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